The `des-cbc-crc`

algorithm (see Cryptographic Overview) uses
the DES key as the initialization vector. This is problematic in
general (see below^{1}), but
may be mitigated in Kerberos by the CRC checksum that is also
included.

From daw@espresso.CS.Berkeley.EDU Fri Mar 1 13:32:34 PST 1996 Article: 50440 of sci.crypt Path: agate!daw From: daw@espresso.CS.Berkeley.EDU (David A Wagner) Newsgroups: sci.crypt Subject: Re: DES-CBC and Initialization Vectors Date: 29 Feb 1996 21:48:16 GMT Organization: University of California, Berkeley Lines: 31 Message-ID: <4h56v0$3no@agate.berkeley.edu> References: <4h39li$33o@gaia.ns.utk.edu> NNTP-Posting-Host: espresso.cs.berkeley.edu In article <4h39li$33o@gaia.ns.utk.edu>, Nair Venugopal <venu@mars.utcc.utk.edu> wrote: > Is there anything wrong in using the key as the I.V. in DES-CBC mode? Yes, you're open to a chosen-ciphertext attack which recovers the key. Alice is sending stuff DES-CBC encrypted with key K to Bob. Mary is an active adversary in the middle. Suppose Alice encrypts some plaintext blocks P_1, P_2, P_3, ... in DES-CBC mode with K as the IV, and sends off the resulting ciphertext A->B: C_1, C_2, C_3, ... where each C_j is a 8-byte DES ciphertext block. Mary wants to discover the key K, but doesn't even know any of the P_j's. She replaces the above message by M->B: C_1, 0, C_1 where 0 is the 8-byte all-zeros block. Bob will decrypt under DES-CBC, recovering the blocks Q_1, Q_2, Q_3 where Q_1 = DES-decrypt(K, C_1) xor K = P_1 Q_2 = DES-decrypt(K, C_2) xor C_1 = (some unimportant junk) Q_3 = DES-decrypt(K, C_1) xor 0 = P_1 xor K Bob gets this garbage-looking message Q_1,Q_2,Q_3 which Mary recovers (under the chosen-ciphertext assumption: this is like a known-plaintext attack, which isn't too implausible). Notice that Mary can recover K by K = Q_1 xor Q_3; so after this one simple active attack, Mary gets the key back! So, if you must use a fixed IV, don't use the key-- use 0 or something like that. Even better, don't use a fixed IV-- use the DES encryption of a counter, or something like that.

[1] The post is copyrighted by David Wagner, included here with permission, the canonical location is http://www.cs.berkeley.edu/~daw/my-posts/key-as-iv-broken